hi,
caustics are always there, only sometimes they get clamped. You can set maximal sample intensity to 0 to get unbiased result, but you will probably get some fireflies. There are other algorithms for rendering caustics, but not in public beta, you'll have to wait for alpha 4 (1-2 weeks).
Use solid glass, thin is for windows.
There is absorption parameter in coronamtl working like vray fog